Brazing

Abstract: The invention provides a method of brazing a first body (10) to a second body (12) and has particular application to the bonding of a composite abrasive compact (10) to a substrate (12). The method involves the steps of providing a brazing shim (18) comprising at least two zones (30, 32) having differing melting temperatures, placing the shim (18) on a surface of the first body (12), placing a surface of the second body (10) on the shim (18) to form an unbonded assembly, applying heat to the assembly to cause the shim (18) to melt and allowing the assembly to cool to a temperature at which all the zones of the shim (18) become solid.

Description: This invention relates to the bonding of bodies together by brazing. When bodies are bonded together by means of induction brazing, it is found that a temperature gradient is set up in the bonding interface. A greater degree of heating takes place at the periphery of the interface than at the centre, so that when a brazing shim is used, the outer or peripheral portion of the shim melts before the inner or central portion. This leads to eventual uneven distribution of the shim material because, on melting, the braze material has a tendency to flow away from the central portion of the shim, often forming a fillet at the edge of the interface. While the fillet may improve bond strength locally, the resultant uneven distribution of braze material overall leads to poor bonding and creates a weak point in the bonded assembly. SUMMARY OF THE INVENTION According to a first aspect of the invention, a method of bonding a first body to a second body comprises the steps of: (i) providing a brazing shim comprising at least two zones having differing melting temperatures, a central zone having the lower of the melting temperatures and an outer zone, surrounding the central zone, having the higher of the melting temperatures; (ii) placing the shim on a surface of the first body; (iii) placing a surface of the second body on the shim to form an unbonded assembly; (iv) applying heat to the assembly to cause the shim to melt; and (v) allowing the assembly to cool to a temperature at which all the zones of the shim become solid. Metals and alloys have a solidus temperature at which the metal or alloy begins to melt and a liquidus temperature at which the metal or alloy is completely molten. In this specification "melting temperature" or "melting point" is used in the same sense as the liquidus temperature of a metal or alloy. In brazing, a metal or alloy is generally heated to a temperature at or above its liquidus temperature and this temperature is known as the working temperature. According to a second aspect of the invention, a brazing shim for use in the above method includes at least two zones having differing melting temperatures, a central zone having the lower of the melting temperatures and an outer zone, surrounding the central zone, having the higher of the melting temperatures. In one preferred form of the invention, the brazing shim has a central zone having the lower of the melting temperatures and an outer zone, surrounding the central zone, having the higher of the melting temperatures. In another preferred form of the invention, the brazing shim has a central zone, an intermediate zone surrounding the central zone and an outer zone surrounding the intermediate zone, the melting temperature of the intermediate zone being higher than that of the central zone, and lower than that of the outer zone. DESCRIPTION OF THE DRAWINGS Figure 1 shows a sectional side view of an induction brazing apparatus which can be used in the brazing method of the invention, and Figure 2 shows a perspective view of an example of a brazing shim according to the invention. DESCRIPTION OF EMBODIMENTS OF THE INVENTION The brazing shim of the invention provides a bonding medium for the bonding together of two bodies. The bodies may be of the same or of different material. In the latter case, the bodies will generally possess different thermal properties. The method of the invention is particularly suited to the bonding of a composite abrasive compact to a substrate. Composite abrasive compacts consist of an abrasive compact layer bonded to a cemented carbide support. The abrasive compact layer may be a diamond compact layer, also referred to as PCD, or a cubic boron nitride layer, also referred to as PCBN. Examples of such composite abrasive compacts are described in United States Patents No. 3,745,623, 3,767,371, 3,743,489, 4,151,686, 4,063,909 and 4,793,828. These United States patents describe composite abrasive compacts in which the abrasive compact layers all contain a second phase which may be metallic or ceramic in nature. Such second phases may be leached away leaving a network of empty pores between adjacent abrasive particles. In bonding such composite abrasive compacts to a substrate, bonding will take place through the carbide support. The substrate will typically also be made of cemented carbide. The cemented carbide for the support and substrate may be any known in the art such as cemented tungsten carbide, cemented tantalum carbide, cemented titanium carbide, cemented molybdenum carbide or mixtures thereof. Bonding takes place between carbide surfaces. The invention may be utilised in bonding a diamond body such as that formed by chemical vapour deposition to a substrate. The substrate may also be a cemented carbide substrate. The shim will be metallic, and will be made of two or more metals or alloys (including active brazing alloys), differing in melting temperatures and each defining a particular zone or area of the shim. There may be a sharp transition between the zones and thus a sharp transition in melting temperature at the boundary between adjacent zones. Alternatively, the transition may be gradual with the alloys of such adjacent zones blending together at the boundary so that the change in melting temperature from one zone to its neighbour is gradual. Where it is desired that the entire shim should melt more or less simultaneously across an interface between two bodies being bonded,the zone of the higher melting temperature should be located in a region which will heat up quicker than other regions. The component alloys or metals of the shim may thus be chosen to suit the heat flux profile for the particular bodies being bonded. The metals or alloys can be chosen to melt in any desired sequence. The brazing shim will generally be produced in sheet form and may be shaped to follow the contour of the bonding interface between the two bodies being bonded. The brazing shim may have a size and shape matching that of the bonding interface. Examples of typical shapes, in plan, are disc, rectangular, triangular and irregular. When it melts, the shim forms an interfacial molten layer and, on cooling and solidification, provides a strong bond securing the bodies together. In the case where the shim is in the form of a flat disc, the zones of differing temperature may be concentrically arranged about a central axis. The outermost or peripheral zone will have the highest melting temperature. The zone adjacent to it will have a lower melting temperature. Progressing toward the centre, each successive zone will have a lower melting temperature. An embodiment of the invention will now be described with reference to the accompanying drawings. Referring to Figure 1, a composite abrasive compact 10 is shown being bonded to a right-circular cylindrical support pin 12 using a brazing shim 18 and in an induction brazing process. The support pin 12 is typically made of cemented carbide. The compact 10 is right-circular cylindrical in shape and has a polycrystalline diamond (PCD) abrasive compact layer 14 bonded to a cemented carbide support 16. The brazing apparatus includes a concentrator plate 20 (usually made of copper), above which are located the rings 22 of an induction heating coil. The innermost ring is in electrical contact with the concentrator plate. A plunger 24 exerts downward force on the composite compact 10 during the brazing cycle. In the brazing process, the shim 18 is placed on the top surface of pin 12 and the lower surface of support 16 brought into contact with the shim 18, as shown in Figure 1. The plunger is lowered into contact with the diamond compact layer 14 and the induction coil is energised. The electrical current flowing through the rings 22 sets up a magnetic field which passes through the workpiece components - the compact 10, shim 18 and support pin 12. Inductive heating takes place first in the peripheral region 28 of the workpiece components within the dotted lines 34 which indicate generally the magnetic field lines. Thus, the shim 18 heats up faster in its peripheral zone 30, while the central zone 32 heats up more slowly. By making the peripheral zone 30 of a higher melting temperature material, melting thereof can be delayed until the central zone 32 has reached its melting temperature or even a higher temperature. When the shim has melted sufficiently with the metals or alloys of the respective zone having reached their working temperatures, heating is stopped. Cooling follows and this allows the metals or alloys of the shim to solidify and form a strong bond between the support pin 12 and the carbide support 16 of composite compact 10, thereby producing a tool insert which may be secured to a tool, such as a drill bit. Figure 2 shows a three component shim 40 in circular disc form. The central zone 42 is made of an alloy or metal with a relatively low melting point. Adjacent to it and surrounding it is an intermediate zone 44 of an alloy or metal of higher melting point. Surrounding this zone 44 is an outer zone 46 of a metal or alloy having the highest melting point of the three components. Depending on the brazing requirements for a particular assembly of unbonded components, the braze shim can include irregularly shaped regions of different melting temperatures. The following examples illustrate the invention: EXAMPLE 1 A 20mm diameter metallic shim was prepared from two alloys. An inner 10mm diameter centre was made of DBF1, a braze alloy having a liquidus temperature of about 915°C and consisting of the following composition by mass: Cu 53%; Au 14,5%; Ni 3,5% and Mn 29%. Surrounding the central alloy and in contact with it, was a ring of Palnicurom 10 (Palnicurom is a trade mark of Wesgo/GTE of Belmont, CA) having a liquidus temperature of about 1013°C and the following composition by mass: Au 25%; Ni 10%; Pd 15%; Mn 13% and Cu 37%. The shim was used to bond the carbide support of a composite diamond abrasive compact sold under the trade name Syndrill to a cemented tungsten carbide substrate containing 13% by mass of cobalt. Both compact and substrate were of right-circular cylindrical shape with outer diameter of 20mm. The compact was placed in the holder in a conventional induction brazing apparatus as shown in Figure 1. Heat was applied in a reducing atmosphere until the outer region of the shim was seen to melt, after which, the assembly was allowed to cool and the shim solidify. The brazing cycle time was 30 seconds. A strong bond was obtained between the carbide support of the Syndrill and the carbide substrate. The bonded assembly was subjected to ultrasonic scanning. The image produced showed no discontinuities across the interface. The assembly was thereafter sectioned diagonally and the interfacial region examined. It was noticed that the entire shim had melted and resolidifed to form a continuous bond across the interface. EXAMPLE 2 A three alloy component shim as illustrated by Figure 2 was prepared by pressing together a central disc and two outer concentric rings. Their characteristics were as follows: Alloy Liquidus T [°C] Composition [Mass%] Central disc Degussa 6488 770 64 Ag 26 Cu 6 In 2 Ni 2 Mn Inner ring Degussa 21/80 990 86 Cu 2 Ni 12 Mn Outer ring Copper only 1100 100 Cu Using the above shim, a 65mm diameter tungsten carbide disc was bonded to a 65mm diameter tungsten carbide disc by means of high frequency induction brazing. Again, ultrasonic scanning and visual inspection showed a continuous bond across the interface. EXAMPLE 3 A further three component braze shim was prepared with a diameter of 38mm and was made up as follows: Alloy Liquidus T [°C] Composition [Mass%] Central disc DBF1 915 53 Cu 14,5 Au 3,5 Ni 29 Mn Inner ring Degussa 21/80 990 86 Cu 2 Ni 12 Mn Outer ring Degussa 97/3 1100 97 Cu 0,6 Si 2,4 Ni The shim was used to bond together by high frequency induction brazing a round Syndrill blank 38mm in diameter to a tungsten carbide post. Ultrasonic imaging and visual inspection of the sectioned interface showed no evidence of discontinuous bonding. EXAMPLE 4 A 10mm square piece of a PCBN composite sold under the trade name DBC50 was brazed to a tungsten carbide substrate using high frequency induction brazing in an argon atmosphere and the following two component shim 10mm square: Alloy Liquidus T [°C] Composition [Mass%] Central square Nicuman 37 925 52,5 Cu 9,5 Ni 38 Mn Outer square Cocuman 999 58,5 Cu 10 Co 31,5 Mn Ultrasonic imaging and visual inspection of the sectioned interface showed no evidence of discontinuous bonding. EXAMPLE 5 A 25mm equilateral triangle of silicon nitride was brazed to a stainless steel support under vacuum in a high frequency induction furnace having an external induction coil using the following two component shim: Alloy Liquidus T [°C] Composition [Mass%] Central triangle Cusil-ABA 815 35,25 Cu 63 Ag1,75 Ti Outer triangle Copper-ABA 1024 92,75 Cu 2 Al 3 Si 2,25 Ti Ultrasonic imaging and visual inspection of the sectioned interface showed no evidence of discontinuous bonding.

Claims: A method of bonding a first body (10) to a second body (12) comprising the steps of: (i) providing a brazing shim (18) comprising at least two zones (30, 32) having differing melting temperatures, a central zone (32) having the lower of the melting temperatures and an outer zone(30), surrounding the central zone (32), having the higher of the melting temperatures; (ii) placing the shim (18) on a surface of the first body (12); (iii) placing a surface of the second body (10) on the shim (18) to form an unbonded assembly; (iv) applying heat to the assembly to cause the shim (18) to melt; and (v) allowing the assembly to cool to a temperature at which all the zones of the shim (18) become solid. A method according to claim 1 wherein the two bodies (12, 16) are made of the same material. A method according to claim 2 wherein the two bodies (12, 16) are made of cemented carbide. A method according to claim 3 wherein at least one of the cemented carbide bodies (16, 12) has an abrasive compact layer (14) bonded to it. A method according to claim 1 wherein the bodies (10, 12) are made of different materials. A method according to claim 5 wherein one of the bodies (10, 12) is made of cemented carbide. A method according to claim 6 wherein the cemented carbide body (16) has an abrasive compact layer (14) bonded to it. A method according to any one of claims 1 to 7 wherein the brazing shim (40) comprises a central zone (42), an intermediate zone (44) surrounding the central zone (42) and an outer zone (46) surrounding the intermediate zone (44), the melting temperature of the intermediate zone (44) being higher than that of the central zone (42), and lower than that of the outer zone (46). A method according to any one of the preceding claims wherein the brazing shim (18, 40) has a size and shape matching the interface between the two surfaces being bonded. A method according to any one of the preceding claims wherein there is a sharp transition in melting temperature at the boundary between adjacent zones (30, 32). A method according to any one of claims 1 to 9 wherein there is a gradual transition in melting temperature at the boundary between adjacent zones (30, 32). A brazing shim (18) for use in a method according to any one of the preceding claims including at least two zones (30, 32) having differing melting temperatures, a central zone (32) having the lower of the melting temperatures and an outer zone (30), surrounding the central zone (32), having the higher of the melting temperatures. A brazing shim (40) according to claim 12 which has a central zone (42), an intermediate zone (44) surrounding the central zone (42) and an outer zone (46) surrounding the intermediate zone (44), the melting temperature of the intermediate zone (44) being higher than that of the central zone (42) and lower than that of the outer zone (46). A brazing shim (18) according to claim 12 or claim 13 wherein there is a sharp transition in melting temperature at the boundary between adjacent zones (30, 32). A brazing shim (18) according to claim 12 or claim 13 wherein there is a gradual transition in melting temperature at the boundary between adjacent zones (30, 32).

Inventors: TANK KLAUS, TANK, KLAUS
Applicants: DE BEERS IND DIAMOND, DE BEERS INDUSTRIAL DIAMOND DIVISION (PROPRIETARY) LIMITED
Application year: 1994
Main classification: B23K1
IPC codes: B23K1, B23B27, B23K35, B23K13, B24D3, B22F7, C04B37